In receivers of a wireless system, typified by cellular phones, a signal received at an antenna is amplified by an amplifier circuit at an initial stage. Such an initial-stage amplifier circuit is required to have characteristics of achieving low noise and high gain when receiving a weak signal, while achieving low distortion and low gain when receiving a large signal. Particularly, in recent mobile communications, since the characteristics of a reception electric field are greatly varied in accordance with a distance between a base station and a mobile station, a wider dynamic range is required than ever in a receiving system. Moreover, an amplifier circuit at a reception front end requires stricter low noise characteristics than ever.
In order to stabilize the operation of the amplifier circuit, a scheme widely used is inserting a resistance between a signal line and the ground at the input or output of the amplifier circuit. However, insertion of the resistance at the input side causes severe degradation in noise characteristics. Insertion of the resistance at the output side, on the other hand, causes sever degradation in distortion characteristics. Other known schemes for stabilizing the operation of the amplifier circuit include a scheme of applying negative feedback to the input. That is, in this scheme, a signal shifted in phase by 180 degrees is fed back to the input. According to this scheme, it is possible to achieve an amplifier circuit generally having a wide dynamic range, although noise and distortion characteristics are both slightly degraded. Better still, a negative feedback circuit used in this scheme can also operate as a distortion compensator circuit, thereby making the dynamic range still wider with some contrivances of the circuit configuration.
Hereinafter, with reference to FIGS. 12 through 16, five conventional negative feedback amplifiers are exemplarily described. A first conventional example is a “negative feedback power amplifier” disclosed in Japanese Patent Laid-Open Publication No. 10-22751 (1998-22751) (refer to FIG. 12). The amplifier illustrated in FIG. 12 includes a negative feedback circuit which is capable of operating as a distortion compensator circuit and is used in a microwave band. In FIG. 12, transistors 601 and 602 are both field-effect transistors. Inductors 603, 604, and 605, and capacitors 606 and 607 form a matching circuit for the transistors 601 and 602. A microstrip line 608 serves as a phase shifter. A power voltage Vcc is applied via the microstrip line 608 to the amplifier.
Part of an output signal from the transistor 602 is fed via the inductor 605, the microstrip line 608, and then the inductor 604 back to the input of the transistor 602. Here, a length L of the microstrip line 608 is adjusted so that the feedback signal and the output signal from the transistor 602 are different in phase from each other by 180 degrees. With part of the output signal including a distortion component being inverted in phase for feedback to the input, distortion characteristics in a high frequency band can be improved.
A second conventional example is a “high-output amplifier” disclosed in Japanese Patent Laid-Open Publication No. 6-216670 (1994-216670) (refer to FIG. 13). The amplifier illustrated in FIG. 13 includes strip lines 701a and 701b as signal lines, a signal-amplifying transistor 702, a directional coupler 703, a feedback strip line 704, a stub 705, resistances 706a and 706b for changing the amount of feedback, a level detector circuit 707, a harmonic suppression controller circuit 708, and a terminator resistance 709.
In FIG. 13, an input supplied via the signal-line strip line 701a is amplified by the signal-amplifying transistor 702. An output from the transistor 702 is fed, via the feedback strip line 704 having a predetermined line length and then the directional coupler 703, back to the input of the signal-amplifying transistor 702 for signal amplification. With this, what is fed back to the input of the signal-amplifying transistor 702 is a signal opposite in phase to a second harmonic. As such, with the distortion of the second harmonic being cancelled, the linearity of the signal-amplifying transistor 702 can be improved.
A third conventional example is an “amplifier” disclosed in PCT International Publication No. WO96/25791 (refer to FIG. 14). The amplifier illustrated in FIG. 14 includes a transistor 801, signal sources 802 and 803, a signal source resistance 804, an input matching circuit composed of components denoted by 805, 806, 807, 808, and 809, an output matching circuit composed of components denoted by 810, 811, and 815, a band-pass filter 812, a phase shifter 813, a variable attenuator 814, and a load resistance 816.
In FIG. 14, the band-pass filter 812 passes a second harmonic of an output from the transistor 801. The phase shifter 813 and the variable attenuator 814 adjust the phase and amplitude of the second harmonic, respectively, In this amplifier, as with the second conventional example, the second harmonic of the output signal is fed back to the input, thereby reducing the third-order intermodulation product of the amplifier.
A fourth conventional example is a “power amplifier” disclosed in Japanese Patent Laid-Open Publication No. 7-94954 (1995-94954) (refer to FIG. 15). The amplifier illustrated in FIG. 15 includes a combiner 901, a power amplifier 902, a divider 903, a filter 904, a variable phase shifter 905, and a variable attenuator 906. This amplifier phase-shifts a fundamental wave and higher order waves (a second or third-or fourth-order harmonic, etc.) of the output from the power amplifier 902 by 180 degrees in a wide band, and the resultant wave is fed back to the input of the power amplifier 902. As such, with the fundamental wave and the second harmonic of the output from the power amplifier 902 being negatively fed back to the input, distortion of the output signal can be compensated.
A fifth conventional example is a “wide-band feedback amplifier” disclosed in Japanese Patent Laid-Open Publication 10-335954 (1998-335954) (refer to FIG. 16). The amplifier illustrated in FIG. 16 includes an amplifying device 1001, a signal input 1002, a signal output 1003, a slot line ground plane 1004, a slot line open plane 1005, a strip line 1006, a microstrip line 1007 in a lot line conversion part, a slot line 1008, via holes 1009, through holes 1010, and a resistance 1011 for determining the amount of feedback. As with the fourth conventional example, this amplifier shifts the phase of the output from the amplifying device 1001 by 180 degrees in a wide band, and then feeds the results back to the input of the amplifying device 1001. As such, with the fundamental wave and the second harmonic of the output from the amplifying device 1001 being fed back to the input, distortion of the output signal can be compensated. The gazette of this publication discloses a specific example of a feedback circuit for phase shift by 180 degrees in a wide band.
However, the above first through fifth conventional examples have the following drawbacks. In the amplifier of the first conventional example, the second harmonic is shifted in phase by almost 360 degrees for feedback to the input. Therefore, this amplifier does not perform distortion compensation by negative feedback of the second harmonic. In the amplifiers of the second and third conventional examples, the fundamental wave is hardly fed back. Therefore, these amplifiers do not perform distortion compensation by negative feedback of the third-order intermodulation wave occurring at a frequency in the vicinity of the frequency of the fundamental wave. Furthermore, in the amplifiers of the fourth and fifth conventional examples, a process of phase adjustment made to both of the fundamental wave and the harmonic is performed only by the feedback circuit, thereby making the feedback circuit complex in structure and increased in size.